Monday, 24 January 2011

Wide angle: 8mm vs 9mm

The Olympus Zuiko 9-18mm f/4-5.6 Four Thirds lens can also be used on Micro Four Thirds cameras, given that you have the appropriate adapter, e.g., Panasonic DMW-MA1, Olympus MMF1 or Olympus MMF2. These are all functionally similar. This lens will autofocus on Micro Four Thirds cameras, but the focus can be a bit slow.

In this article, I am comparing the first and the last on this list. Here's a picture of them both:

The Olympus Zuiko 9-18mm f/4-5.6 (left) is shown without the appropriate adapter. Mounting the adapter will add 18.67mm length, since that is the difference between the register distance of the two formats.

The Panasonic Lumix G 8mm f/3.5 fisheye is a truly compact lens. Olympus has a similar lens on their 2011 roadmap, and it remains to see how compact it will become.

Field of view

At 9mm focal length, the Olympus lens has a diagonal field of view of 100º. The 8mm fisheye lens, on the other hand, has a diagonal field of view of a whopping 180º! How can one mm difference in focal length make up such a massive difference in field of view?

The answer is that the projection is different in the two lenses. Projection in this case refers to the mapping of the real world objects in three dimensional space, down to the image sensor and two dimensions.

Most photographic lenses feature a rectilinear projection. This is what we have become used to. A rectilinear lens will produce an image where straight lines in the real world object are straight also in the resulting image.

Fisheye lenses are fundamentally different. With a fisheye lens, only straight lines the pass through the image centre are straight. All other lines will be bent. There is a significant amount of barrel distortion.

Within fisheye lenses, there can also be variations. Circular fisheye lenses will give a disc of exposure. A 180º view in all directions is mapped into a single disc, and the rest of the sensor frame is left black.

Full frame fisheyes are perhaps more common. They feature a 180º only in the diagonal, and otherwise fill out the entire sensor area. The Lumix G 8mm f/3.5 is a full frame fisheye lens. To most users, these are more useful, as they give a rectangular image, as we are used to.

Example images

To further illustrate the difference between a rectilinear wide angle lens and a fisheye lens, let's look at an example. The images below were taken at base ISO, and on a tripod.

Olympus 9-18mm @ 9mm f/4

Lumix G 8mm Fisheye @ f/3.5

Olympus 9-18mm @ 9mm f/8

Lumix G 8mm Fisheye @ f/8

As you can see, the fisheye image is wider, and also features significant barrel distortion. Straight lines in the real objects are bent in the depiction.

This is not really the right type of image to evaluate the vignetting, but it seems that the 8mm fisheye lens vignettes a bit more at f/3.5. However, with such a wide angle of view, it is unlikely that you have the same tone across the field anyway, so I cannot see that vignetting will be a significant issue with this lens.

Note that the light source, the setting sun, is in the middle of the frame. Both lenses handle this fairly well. There is not a big amount of flare or lack of contrast caused by the light source in the centre of the frame.

Sharpness

To evaluate the sharpness, let's look at 100% crops from various parts of the image. These images have not been sharpened. Click for a larger version.

Here are crops from the centre of the frame:

We see quite clearly that the Lumix 8mm Fisheye is the sharpest lens, straight from wide open at f/3.5.

Corners and Chromatic Aberration

From these border crops, we see basically the same thing. We can see some softness in the 8mm Fisheye image at f/3.5, but it sharpens up well at f/5.6.

Also, we see some Chromatic Aberration lens distortion artifacts in the Olympus images. There's the red and green fringing off high contrast areas. These artifacts typically appear near the borders, and become more significant the further away from the image centre you get. This can be corrected pretty well by software, so it's not a big issue.

Panasonic lenses are automatically corrected for Chromatic Aberration (CA) distortion during the in-camera image processing, when using Panasonic cameras. So the JPEG out of camera images I have used could have been corrected for these effects, which may be why we don't see any CA in the Fisheye images.

To examine the effects of the automatic CA correction in the 8mm fisheye lens, let's look at one example. This picture of the Morris Jumel Mansion was taken with the GH1 and the Lumix G 8mm f/3.5 Fisheye:

Here are 100% crops from the extreme top right corner, and from the middle right border:

We see that the corrected image still shows some colour fringing artifacts in the extreme corner, but they are mostly gone in the border crop. From the rest of the frame, i.e., not the extreme borders, you will be hard pressed to find any CA artifacts in the out of camera JPEG image.

From the RAW images which have not been corrected for CA distortion, we see that there are some CA artifacts. These fringes are about 1-2 pixels wide in the extreme corner, which is not very significant. For comparison, the colour fringes are about 2-3 pixels wide in the images from the Olympus Zuiko 9-18mm f/4-5.6 wide zoom lens.

It's also fair to comment that the image from the Lumix 8mm fisheye is remarkably sharp in the extreme corner. Keep in mind that the corner is at a 90º angle from the optical axis.

Defishing

It is possible to convert the fisheye image to a normal image. This process is usually refered to as defishing the image.

Many different programs allow this kind of transformations. I have used a program called Hugin for this purpose.

Below is a comparison of the original images taken with the rectilinear lens at 9mm, the fisheye, and, in the bottom row, the fisheye image converted to rectilinear.

Olympus 9-18mm @ 9mm f/8

Lumix G 8mm Fisheye @ f/8

Fisheye image converted to rectilinear

Fisheye image converted to rectilinear and cropped

The original fisheye image can be stretched to look fairly similar to the rectilinear image. But it has an even wider field of view. I would say the difference in field of view is significant.

This defishing process is hardly optimal, though. The corners have been stretched, and hence lack some resolution compared with the centre of the image. Also, it is difficult to frame the image correctly if you intend to defish it later. But having the option to defish the image makes the fisheye lens more useful.

Here is another example of defishing. The original image is from the Apple Center in New York, Manhattan:

After defishing, it looks like this. There is still a bit of barrel distortion, which could have been removed with some tweaking of the parameters. You can see that the image is less sharp in the corners, due to the stretching needed in the defishing process.

Another note is that the aspect ratio changes when defishing the image. The original fisheye images were taken with a 4:3 aspect ratio, while the defished image has a much wider aspect ratio, closer to 16:9.

On first inspection, this might look like a mistake. However, it does actually make sense. A fisheye lens creates an image where the field of view is not constant across the frame. What I mean is that the field of view is more compressed in the corners than in the centre. Hence, the ratio of horizontal to vertical field of view becomes larger than that of the original image.

When using the 4:3 aspect ratio, the output image of course has a 4:3 ratio in terms of pixels. However, due to the compressed field of view in the corners, the horizontal to vertical field of view ratio is 124º:92º.

Conclusion

In concluding, it is clear that the Lumix G 8mm f/3.5 Fisheye lens is better optically than the Olympus 9-18mm zoom. The Lumix 8mm fisheye appears to vignette a bit with wide open aperture, but I can't see that being a big problem.

On the other hand, the 9-18mm zoom is more versatile. In the longer end, it gives a pretty normal field of view, and can be used for general photography. In the wide end, it is an extreme wide angle lens. It can be used to make stunning and interesting wide angle images, as well as pictures with a more normal perspective.

The fisheye lens is an exotic lens, and is not always easy to use. When you nail an interesting image with the fisheye lens, it can be very rewarding. But many pictures end up looking just hideous, or like clichés. It is a lens with a required taste. Given the high price, I would not recommend buying it unless you know what you are doing.

Of course, nobody with a Micro Four Thirds camera should buy the Olympus Zuiko 9-18mm f/4-5.6 Four Thirds lens, since it requires and adapter, and is much larger than the M.Zuiko Micro Four Thirds version of the lens. From what I have read, the M4/3 version has comparable optical qualities, and focuses much faster. It is also collapsible, and much lighter and more compact.